It isn't hard to imagine that professional athletes tend to have healthy blood. Exercise takes a lot of oxygen. Red blood cells carry oxygen, so the body makes more red blood cells to carry more oxygen the more athletes exercise. The oxygen-carrying molecule hemoglobin does double duty for athletes. Not only do high hemoglobin levels result in greater oxygen-carrying capacity for the bloodstream, they also cause the release of a chemical called nitric oxide (NO) that dilates blood vessels so more oxygen can be carried to muscles. 
How does exercise increase red blood cell count?
Red blood cells are made by the bone marrow. Exercise causes bone marrow to grow. It also induces the release of hormones that increase the production of hemoglobin.  Athletes who train under conditions that require even more oxygen, such as training at high-altitude locations, develop even more red blood cells with even more hemoglobin .
Do exercise and running increase red blood cell count for people who with health challenges?
But what about those of us who aren't athletes, who can't afford to spend a summer in Aspen working out, who don't have free days just for working out? It turns out that exercise increases red blood cell count for people who struggle with health challenges as well as those who enjoy athletic prowess. Here are some examples.
- In one study women who had rheumatoid arthritis had higher hemoglobin levels, higher red blood cell mass, and greater oxygen-carrying capacity of the blood after a program of exercise that lasted just eight weeks and that only required them to exercise at a moderate rate. There were not enough women enrolled in the study for the results to achieve statistical significance, but even with this study of just 17 women, the trend showed that the health of the blood improved with exercise despite arthritis. 
- It isn't necessary to put out all the effort for running to increase red blood cell count. In another study, men with obesity were found to have at least a short-term increase in red blood cell count and hematocrit (percentage of the blood that is red blood cells) immediately after exercise. The greatest increase in red blood cells occurred after lower-intensity exercise than higher-intensity exercise, but with fewer breaks rather than more breaks. Slow, steady exercise did the most to trigger higher hemoglobin levels and higher red blood cell counts in obese men. 
- Heat stress causes higher hematocrit and higher hemoglobin levels even without dehydration . You don't have to work out hard to experience changes in your blood by being exposed to heat, although dehydration, whether you work out hard or not, will accelerate the effect.
- Resistance exercises help people getting chemotherapy for lung cancer to maintain white blood cell counts that help them fight infections. (The type of resistance exercise studied was done with elastic bands.)  And at least 16 studies have found that aerobic exercise (walking, stationary bike, swimming, and so on) helps red store red blood cell counts lowered by chemotherapy, especially women getting chemotherapy for breast cancer .
Diet makes a difference in the way your blood cells respond to exercise
Zinc deficiencies interfere with the action of hemoglobin. If you get enough zinc, you will breathe better when you exercise.  This doesn't mean that if you get more and more zinc you will have more and more exercise capacity. It just means that you need to avoid zinc deficiency. Generally, if you take a zinc supplement and you don't notice an aftertaste, you probably need it. However, because zinc interferes with your body's ability to absorb copper, don't take more than 30 mg a day on a regular basis, and take 1 to 3 mg of copper along with your zinc supplement.
Exercisers who use whey powder tend to develop higher red blood cell counts and higher hemoglobin levels than those who do not. They also experience less fatigue after exercise.  Although they are specialty foods, black cumin seeds and lotus seeds contain chemicals that help your body use the iron to make hemoglobin and possibly influence hormones involved in red blood cell production.
The important thing to remember is that almost any level of exercise helps your body to make the red blood cells it needs to carry oxygen through your bloodstream. The more you exercise, the more your body will use the iron in your diet and supplements . If you are not capable of high-intensity exercise, that's OK, slow and steady exercise is enough to stimulate your body to produce more of the red blood cells it needs.